Offshore oil production normally involves secondary oil recovery for economical overall production; however, before an oil formation may be flooded by means of a high pressure water jet, the water to be injected should meet certain requirements. Such water should be free from suspended solids and essentially free from sulfate in order to eliminate potential oil reservoir blockage which can be caused by such solids and by less soluble ions, e.g. barium, reacting with sulfate that might be carried in any significant concentration in such injection water. Furthermore, it is important that the injection water should have a similar ionic concentration to that of the oil reservoir water, e.g. chloride ion content, lest clay or other minerals in the formation would be caused to swell as a result of significant difference in ionic strength in water being injected during oil production operation.
The prior art contains processes for water treatment for use in such oil production. Generally, these prior art processes involve the use of softeners, ultrafiltration and/or other membrane techniques in an attempt to provide water having a high chloride ion concentration and a low sulfate iron concentration. British Patent No. 1,520,877 (UKAEA) (Aug. 9, 1978) describes a process for the secondary recovery of oil where treated water is injected at a high pressure through an injection well. Prior to injection, raw sea water, after first treatment with an ultrafiltration apparatus, is fed to a first reverse-osmosis apparatus at a pressure of about 1,000 psi and thereafter to a second reverse-osmosis apparatus at a pressure of about 1,500 psi to produce a final product water concentrate which is utilized. A subsequent British patent issued to the UKAEA, No. 1,522,595 (Aug. 23, 1978) describes a two-step injection process wherein water of a relatively low ionic concentration is first injected into the oil reservoir through the injection well, causing the pores of clay-like particles to gradually close until pores of the rock near the outlet well become filled with oil. Thereafter, water having the predetermined higher ionic concentration is injected, causing the clay-like particles to shrink so that water can flow through the spaces and force the oil to the outlet well.
These documents teach that, by treating the injection water source by ultrafiltration followed by two sequential reverse-osmosis treatments, using desalination units that were state-of-the-art at that time, it was possible to adjust the ionic composition to desirable levels. Also contained is the teaching that certain divalent ions, e.g. sulfates, react with counterions, e.g. barium ions, found in the rock reservoir to form insoluble sulfates, the result of which can be pore blockage as a result of precipitation of such sulfates. The intention appears to be to adjust the ionic composition to remove divalent runs, but no explanation is given as to how such removal is effected. The initial treatment by ultrafiltration is said to remove minute particles less than a micron in diameter, and cellulose acetate RO membranes are used to adjust the composition of the concentrate which is the desired end product.
U.S. Pat. No. 4,723,603 to Plummer (Marathon Oil Company) issued Feb. 9, 1988 took advantage of subsequent developments in separation membrane technology to provide a more efficient process for treating water for use in secondary oil recovery by removing precursor ions, i.e. bivalent anions and cations, from the water being injected while keeping a desired concentration of soluble monovalent ions in the water. Such was accomplished using a polyamide semipermeable membrane of the nanofiltration type sold under the tradename FilmTec NF-40. The membrane was of the reverse-osmosis type, but it was alleged to effect an adequate rejection of bivalent ions, such as sulfate, at 22° C. while allowing about 91-92% of monovalent ions, e.g. sodium, chlorine, etc., to pass in order to effect the clay stabilization taught to be desirable in the aforementioned British patent.
Although the process taught in the '603 patent was worthwhile in reducing two RO treatment-steps to one membrane treatment step, it too experienced significant limitations. It was often unable to achieve desired concentrations in a single step, and it was not able to operate in warm or tropical regions (where the temperature of the water being treated is substantially higher than that found in the North sea and other drilling locations where offshore oil production was occurring) without expensive cooling of the feedwater. Therefore, improved processes continue to be sought.